Many studies have been made of computer-to-plate printing plate, which has been remarkably developed in recent years. Under these circumstances, aiming at further process rationalization and solution to waste liquid problems, a lithographic printing plate precursor which can be mounted on the printing machine free from development after exposure for printing purpose and a lithographic printing plate precursor which can be exposed to light on the printing machine shortly before printing have been studied. Various related methods have been proposed.
For example, Japanese Patent 2,938,397, Japanese Patent Laid-Open No. 1997-127683 and WO99-10186 disclose a heat-sensitive lithographic printing plate precursor comprising a hydrophilic image-forming layer having a particulate thermoplastic polymer dispersed in a matrix such as hydrophilic resin provided on a substrate having a hydrophilic surface. These patents disclose that the conventional development process involving the use of an automatic developing machine can be omitted by using a method (on-the-machine development method) which comprises removing unheated area as if it has been developed, i.e., applying heat to the image-forming layer by exposure to infrared rays or the like so that the particulate thermoplastic polymer undergoes melt-coalescence to convert the surface of the hydrophilic image-forming layer to a hydrophilic image area, mounting the lithographic printing plate having this image area formed thereon on the printing machine, and then supplying fountain solution and ink onto the printing plate while rotating the plate cylinder.
Further, Japanese Patent Laid-Open No. 2000-238452 discloses that a lithographic printing material comprising a microgel having a group which decomposes by at least one of heat and heat energies on the surface thereof and an infrared-absorbing agent incorporated in an image-forming layer can be subjected to development on the printing machine.
However, the aforementioned on-the-machine type unprocessed lithographic printing plate precursor is disadvantageous in that it requires much cost and time. For example, since the removal of the unexposed area is governed by the conditions under which the printing machine begins to operate and the material containing much hydrophilic components thus removed contaminate water roller and fountain solution, scores or hundreds of sheets need to be wasted until good printed matters are obtained or the roller must be cleaned.
Research Disclosure No. 33303, January 1992, discloses a heat-sensitive lithographic printing plate precursor having a heat-sensitive layer comprising a particulate thermoplastic polymer dispersed in a crosslinked hydrophilic resin. Further, Japanese Patent Laid-Open No. 1995-1849, Japanese Patent Laid-Open No. 1995-1850, Japanese Patent Laid-Open No. 1998-6458 and Japanese Patent Laid-Open No. 1999-70756 disclose a heat-sensitive lithographic printing plate precursor having a hydrophilic layer comprising microcapsules having a hydrophilic component encapsulated therein as hydrophilic particles dispersed in a crosslinked hydrophilic binder polymer. It is also disclosed that these heat-sensitive lithographic printing plate precursors comprise as a printing surface a surface composed of a hydrophilic image area formed by heat developed by exposure and an unexposed hydrophilic non-image area and thus require no development on the printing machine, allowing lithographic printing with fountain solution without processing.
However, the aforementioned related art lithographic printing plate precursor which requires no processing leaves something to be desired in stain resistance during printing and press life.
It is known that a particulate metal oxide (e.g., siO2, TiO2) is used as a hydrophilic material to improve background stain resistance. Japanese Patent Laid-Open No. 2000-79771 discloses the use of a particulate metal having a size of not greater than 100 nm. This particulate metal is sufficiently hydrophilic on the inorganic surface thereof and has an enhanced surface roughness and hence a raised water retention to improve the background stain resistance of the lithographic printing plate precursor. On the other hand, however, a film containing a metal oxide dispersion is subject to cracking (fine cracking occurring during drying). An ordinary method for preventing the occurrence of cracking is to add PVA (polyvinyl alcohol) as a binder. However, the use of PVA causes deterioration of hydrophilicity, causing the lithographic printing plate precursor subject to background stain when it is squeezed out of fountain solution. Therefore, it is the status of quo that no methods have been obtained for obtaining a sufficient hydrophilicity free from adverse effects on the physical properties of film.
The inventors made studies of solution to these problems. As a result, it was found that the combined use of a hydrophilic polymer terminated by a silane coupling group and a particulate metal oxide causes the hydrophilic polymer to be selectively grafted on the surface of the printing plate precursor, making it possible to prevent the occurrence of background stain. It was further found that the film of hydrophobicizing resin particle which has been converted to a hydrophilic image area by heating can be kept ink-receptive and exhibits an excellent press life. However, since a dehydration condensation reaction is employed to harden the hydrophilic polymer-containing binder layer having a silane coupling group (silica sol-gel), a certain acidic catalyst or basic catalyst is required to obtain a highly hydrophilic and hard microphase separation structure. When an acidic or basic catalyst is added to the coating solution to an extent such that a catalytic effect can be exerted, it is disadvantageous in that the age stability of the coating solution is deteriorated or the conditions of the coated surface are defective (deterioration of smoothness).
As shown in the aforementioned background of the related art technique, an attempt to satisfy excellent press life and print quality such as background stain resistance resulted in disadvantages of coat quality such as drop of producibility such as age stability of coating solution and smoothness of coated surface. Satisfactory status of quo have never been reached.